1. Field of the Invention
This invention concerns improved processes for the synthesis of H.sub.2 O.sub.2 by means of the electrochemical reduction of oxygen in the presence of an acid and halide, preferably bromide, in an electrolytic cell or in the cathode compartment of a fuel cell. The processes described herein permit production of hydrogen peroxide in commercially useful concentrations.
2. Background Art
The literature is replete with publications concerning the reduction of oxygen to water or to H.sub.2 O.sub.2. The extensive list of the publications is at least partially due to the interest in developing fuel cells. However, the goal of the fuel cell work has been the direct reduction of oxygen to water without the intermediate production of H.sub.2 O.sub.2. The problem addressed herein is the maximization of the production of H.sub.2 O.sub.2. One persistent problem associated with the production of H.sub.2 O.sub.2 by the electrochemical reduction of oxygen is that product yields have been too low for use in commercial applications. Although various improvements have been made in the direct synthesis of H.sub.2 O.sub.2 from H.sub.2 and O.sub.2 by non electrochemical processes employing various catalysts, there is a dearth of literature addressing synthesis of H.sub.2 O.sub.2 by means of electrochemical processes.
Fischer and Priess [Ber. 46, 698 (1913)] studied the reduction of O.sub.2 to H.sub.2 O.sub.2 at a gold sheet cathode.
M. S. Tarasevich, A. Sadowski and E. Yeager in "Comprehensive Treatise of Electrochemistry" Vol. 7, Ed. by B. E. Conway, J. O. M. Bockris, S. U. M. Khan and R. E. White (1983) at page 353 review prior work on the electroreduction of oxygen. Included is reference to a paper in which a ring/disc electrode was used to study the reduction of oxygen on platinum and palladium in acid solutions containing chloride or bromide.
V. S. Vilinskaya and M. R. Tarasevich "Electrokhimiya" Vol. 9, No. 8., 1187 (1973) (p. 1123 in the English Translation) study the effects of various absorbed anions and cations on oxygen reduction and conclude that the increase in surface coverage of the electrode by chemisorbed oxygen and ions leads to a decrease in available sites for the dissociative chemisorption of oxygen.
A review of cathodic reduction of oxygen is in W. C. Schumb, C. N. Satterfield and R. C. Wentworth, "Hydrogen Peroxide", Reinhold Publ. Co., New York, 1955, p. 66.
Advances in Catalytic Technologies: Novel Oxygenation Reactions, Catalytica.RTM. Study No. 4186 (1986), prepared by Catalytica.RTM., 430 Ferguson Drive, Building 3, Mountain View, Calif. 94043 summarizes developments related to the cathodic reduction of oxygen to form hydrogen peroxide. In discussing recent kinetic studies related to the reduction of oxygen on carbon surfaces and the effect of basic and acidic surface groups on this process, it is pointed out that "Not surprisingly, since the unsuitability of acid electrolytes for H.sub.2 O.sub.2 synthesis was demonstrated in early work, its use has been confined to kinetic studies." Id. at p. 13.
U S. Pat. No. 4,772,458 issued to Gosser et al. employs a catalytically effective amount of platinum, palladium or a combination thereof with an aqueous reaction medium comprising an acid component and a bromide promoter wherein the acid and bromide are provided in amounts to provide a molar ratio of hydrogen ion to bromide ion of at least about 2:1 in the reaction medium. Various other catalytic processes aimed at improving the process for the production of hydrogen peroxide by the direct combination of hydrogen and oxygen are discussed in the backgound section of U.S. Pat. No. 4,772,458 at columns one through three. This discussion is incorporated herein by reference.
John J. McKetta and William A. Cunningham, Encyclopedia of Chemical Processing and Design, Marcel Dekker, Inc., New York and Basel, 1986, Chapter 24, pages 1-26, "Fuel Cells" describe a variety of fuel cell structures. This discussion is incorporated herein by reference.
K. Otsuka and I. Yamanaka, One Step Synthesis of Hydrogen Peroxide Through Fuel Cell Reaction, Electrochimica Acta, Vol. 35, No. 2, pages 319-322, February 1990 describes a fuel cell system (O.sub.2, HCl or H.sub.2 SO.sub.4, M cathodes, Nafion-H Pt. anode, H.sub.2) where M is Pt, Pd, Au, graphite or Au-mesh. It was found that less active materials such as Au-mesh were favorable electrodes for the synthesis (See page 320, col. 1).